The present invention relates generally to a signal generator generating test signals and more particularly to a signal generator generating a test signal having emulated crosstalk.
In high speed digital signaling standards, such as PCI-Express, DisplayPort and the like, individual components of a signal channel, such as connectors, cables, circuit board traces and the like, can be represented by S-parameters (scattering parameters). These components can cause impairments in the signal transported across the channel, such as intersymbol interference, noise, crosstalk and the like, that are not desirable. The above standards adopt differential signaling for reducing these impairments.
Crosstalk is caused by capacitive and/or inductive coupling into one signal channel by a signal or signals on an adjacent signal channel or channels. The signal and signal channel being affected by the crosstalk are respectively referred to as the victim signal and victim signal path and the adjacent signal or signals on the adjacent signal channel or channels are respectively referred to as the aggressor signal or signals and aggressor signal path or paths. The crosstalk dominantly affects the victim signal both at the near and far ends of the signal path.
High speed serial data designers are required to design robust and reliable receivers to meet the required bit error rate for specified conditions. Crosstalk from neighboring paths has an adverse effect on the performance of high speed serial data. A designer needs to ascertain the effect of crosstalk, which is dictated by the physical property of signal paths coupled to the receiver, such as the PCB (print circuit board) traces, cables, connectors and the like, and also the characteristic of aggressor and victim signals.
FIG. 1 shows a conventional test setup 10 for measuring the compliance of a DisplayPort communication system to an applied victim signal having crosstalk generated using aggressor signal. A signal generator 12, such as an AWG7122B manufactured and sold by Tektronix, Inc., Beaverton, Oreg., generates a victim signal and aggressor signals that are provided to a DisplayPort device under test (DUT) 14, such as a transmitter/receiver circuit. The victim signal and the aggressor signals are provided to a test fixture 16, such as an ET-DP-TRA-P Plug Fixture, such as manufactured and sold by Efficere Technologies, Vancouver, Wash., via Transition Time Converter (TTC) modules 18. The TTC modules 18 slow the rise time of the differential victim and aggressor signal, to be in compliance with the DisplayPort compliance testing standard. The victim signal and the aggressor signals are provided to the receiver inputs of the DUT 14 via the test fixture 16. The transmitter outputs of the DUT 14 are coupled via the test fixture 16 to a DisplayPort AUX Control 20 that receives the differential transmitter output, coverts the differential signal to a single ended signal and conditions the signal for use by a test and measurement instrument 22, such as an oscilloscope. The DisplayPort AUX Control 20 couples the single ended transmitter signal to an acquisition channel input of the oscilloscope, such as a DPO70254 Digital Oscilloscope, manufactured and sold by Tektronix, Inc., Beaverton, Oreg.
The DisplayPort set-up has four channels shown as Lane 0, Lane 1, Lane 2 and AUX channel. In Receiver Stress Testing, a Test Lane (e.g. Lane 1) is fed with a test signal (victim signal) in the form of a stressed pattern having various signal impairments, such as deterministic and random jitter, noise and the like, from a signal generator 12, such as the AWG7122B manufactured and sold by Tektronix, Inc., Beaverton, Oreg. The signal generator 12 also feeds aggressor signals to the neighboring lanes (Lanes 0, Lane 2) in the form of a half clock pattern or the like, which inflict crosstalk onto the Test Lane. The DisplayPort Standard provides the specification for both the victim signal and the aggressor signal. The layout of the test fixture 16 produces crosstalk on the victim signal on Lane 1 in response to the aggressor signals on Lanes 0 and Lanes 2. The victim signal is coupled to the DUT 14 via the TTC modules 18, associated cabling and the test fixture 16. The stressed test signal with crosstalk is processed by the receiver circuitry in the DUT 14 and coupled to the transmitter circuitry in the DUT 14, which processes the received signal and outputs a transmitter signal via the auxiliary channel to the DisplayPort AUX Control 20. The DisplayPort AUX Control 20 converts the transmitter differential signal to a single ended signal and conditions the signal for use by serial trigger circuitry in the oscilloscope 22. The oscilloscope receives the output signal from the DisplayPort AUX Control 20, digitizes and processes the incoming signal and displays the resultant signal data on a display device for analysis.
The process of setting up for a DisplayPort receiver stress test is cumbersome, time consuming and prone to errors. In addition, the set-up does not allow flexibility for the designer to modify the channel and signal parameters. Further, costs may occur as revisions and improvements of the standard are implemented requiring new cabling, fixtures, and the like. The physical properties of the custom made fixtures determine the influence of crosstalk on the victim signal. It could be much different from the actual hardware signal channels designed for the DUT 14. Therefore, it is important for a designer to test the transmitter/receiver design with crosstalk generated by the actual hardware signal channels.